1. Field of the Invention
This invention relates to the field of automatic landings by aircraft in low visibility conditions. More specifically this invention relates to systems which provide the pilot with visual cues which represent a runway and automatic landing systems which permit guidance and automatic landing by aircraft in low visibility conditions when there is little or no ground aid signals available.
2. Description of the Prior Art
The ILS systems in use today evolved from the early use of radio frequency beams installed at the airport to provide beam guidance for aircraft to a runway. The beam consists of radio frequencies which emanate from ground based antennas with the radiated fields overlapping so that with equal strength of each of the radiated fields an approximate straight line is established. A localizer and glideslope set of antennas (on-board the aircraft) are required.
This system comprises the Instrument Landing Systems (ILS) that have been installed at airports throughout the world. The desire to fly in all types of weather has led to the growth and development of equipment installed at the airport and on the aircraft which will allow the aircraft to land in various weather levels. Such weather levels are defined as Category I, II, and III. Category I is 2400 or greater feet Runway Visual Range (RVR), Category II is 1200 to 2400 feet RVR and Category III which is subdivided into three levels of IIIa at 1200 to 700 feet RVR, IIIb at 700 to 150 feet RVR and IIIc at 150 to 0 feet RVR.
The implementation of the categorized weather conditions has led to automatic landing systems being used as the primary method of landing an aircraft in all category III weather conditions.
The problems experienced with the ILS as affected by ground structures such as hangar doors, and creation of variability in the beam by the opening and closing of hangar doors, existing buildings being installed or removed in the vicinity of the antenna and even including aircraft taxiing near or flying over the antenna. These beam distortions can be tolerated in some cases and cannot in others. In fact some of the times the beams are shut down in snow and ice conditions. These problems have been severe enough that the FAA has developed a new system over the last 20 years called Microwave Landing System (MLS) that is to replace the ILS. The MLS system is intended to also provide ground based signals for category I, II, and III landing systems for use during inclement weather.
The airlines recognize that some of the limitations of the ILS would be overcome by the MLS system which also sends out sector data information which is more useful to an aircraft than straight line information of the ILS. However, they have questioned the long term economics of this system, due to the expensive on-board equipment required for implementation of the MLS system and the need to replace all of the existing ILS (which are currently being replaced very slowly). Currently there are about 35 airports in the United States that are equipped with the category 3 ILS beams. The FAA has been reluctant to install additional ILS systems in view of the benefits of the MLS and airlines are reluctant to endorse the MLS because of the expense of the airborne equipment and continued dependence on the FAA.
In order to solve the problems of limited number of installations of the ground based category III ILS systems and the rate at which the FAA is installing replacement MLS systems, and also to obtain greater autonomy with the use of the aircraft, it is desirable to have a system which provides a real time sense of the airport for approach and landing on-board the aircraft.
The instant invention does provide such a system, which permits all category III weather landings, does not require ground based signals, does not require receiving antennae, and does not require expensive ground based equipment installation.